Re: [PATCH 1/2] mm/large system hash: use vmalloc for size > MAX_ORDER when !hashdist

[Nicholas Piggin <npiggin@gmail.com>]

Andrew Morton's on June 6, 2019 7:22 am:
> On Thu,  6 Jun 2019 00:48:13 +1000 Nicholas Piggin <npiggin@gmail.com> wrote:
> 
>> The kernel currently clamps large system hashes to MAX_ORDER when
>> hashdist is not set, which is rather arbitrary.
>> 
>> vmalloc space is limited on 32-bit machines, but this shouldn't
>> result in much more used because of small physical memory limiting
>> system hash sizes.
>> 
>> Include "vmalloc" or "linear" in the kernel log message.
>> 
>> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
>> ---
>> 
>> This is a better solution than the previous one for the case of !NUMA
>> systems running on CONFIG_NUMA kernels, we can clear the default
>> hashdist early and have everything allocated out of the linear map.
>> 
>> The hugepage vmap series I will post later, but it's quite
>> independent from this improvement.
>> 
>> ...
>>
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -7966,6 +7966,7 @@ void *__init alloc_large_system_hash(const char *tablename,
>>  	unsigned long log2qty, size;
>>  	void *table = NULL;
>>  	gfp_t gfp_flags;
>> +	bool virt;
>>  
>>  	/* allow the kernel cmdline to have a say */
>>  	if (!numentries) {
>> @@ -8022,6 +8023,7 @@ void *__init alloc_large_system_hash(const char *tablename,
>>  
>>  	gfp_flags = (flags & HASH_ZERO) ? GFP_ATOMIC | __GFP_ZERO : GFP_ATOMIC;
>>  	do {
>> +		virt = false;
>>  		size = bucketsize << log2qty;
>>  		if (flags & HASH_EARLY) {
>>  			if (flags & HASH_ZERO)
>> @@ -8029,26 +8031,26 @@ void *__init alloc_large_system_hash(const char *tablename,
>>  			else
>>  				table = memblock_alloc_raw(size,
>>  							   SMP_CACHE_BYTES);
>> -		} else if (hashdist) {
>> +		} else if (get_order(size) >= MAX_ORDER || hashdist) {
>>  			table = __vmalloc(size, gfp_flags, PAGE_KERNEL);
>> +			virt = true;
>>  		} else {
>>  			/*
>>  			 * If bucketsize is not a power-of-two, we may free
>>  			 * some pages at the end of hash table which
>>  			 * alloc_pages_exact() automatically does
>>  			 */
>> -			if (get_order(size) < MAX_ORDER) {
>> -				table = alloc_pages_exact(size, gfp_flags);
>> -				kmemleak_alloc(table, size, 1, gfp_flags);
>> -			}
>> +			table = alloc_pages_exact(size, gfp_flags);
>> +			kmemleak_alloc(table, size, 1, gfp_flags);
>>  		}
>>  	} while (!table && size > PAGE_SIZE && --log2qty);
>>  
>>  	if (!table)
>>  		panic("Failed to allocate %s hash table\n", tablename);
>>  
>> -	pr_info("%s hash table entries: %ld (order: %d, %lu bytes)\n",
>> -		tablename, 1UL << log2qty, ilog2(size) - PAGE_SHIFT, size);
>> +	pr_info("%s hash table entries: %ld (order: %d, %lu bytes, %s)\n",
>> +		tablename, 1UL << log2qty, ilog2(size) - PAGE_SHIFT, size,
>> +		virt ? "vmalloc" : "linear");
> 
> Could remove `bool virt' and use is_vmalloc_addr() in the printk?
> 

It can run before mem_init() and it looks like some archs set
VMALLOC_START/END (high_memory) there (e.g., x86-32, ppc32).

Thanks,
Nick